1. Field of Invention
The present invention relates to current generating circuits for use in driving display panels, such as organic EL (Electronic Luminescence) panels. More specifically, the invention relates to a current generating circuit to generate current with non-linear characteristics with respect to digital data indicating the brightness of a display panel.
2. Description of Related Art
Generally, in liquid crystal panels, a change in gray level (brightness) of a pixel is not proportional to the voltage applied to the pixel. When driving liquid crystal panels, voltages with non-linear characteristics with respect to the linear gray scale of pixels (generally defined by digital data) are output. Thus, it would appear as if the gray scale changes linearly.
On the other hand, human visual characteristics are generally known as having logarithmic or exponential characteristics. If the brightness, represented as the gray level, changes linearly, it may not appear to the human eye that the gray level changes linearly. In view of these circumstances, an electro-optical device should have logarithmic or exponential gray scale characteristics, and hence it appears to the human eye that the gray scale has linear characteristics. A series of these processes may be referred to as “γ” correction.
Recently, organic EL panels have been attracting interest as next-generation display panels. This is because, unlike liquid crystal elements for simply changing the light transmission, organic EL elements used as electro-optical elements in organic EL panels are self-luminous elements which emit light by themselves. For this reason, organic EL panels have excellent characteristics, such as a wider viewing angle, a higher contrast, and a higher response speed than liquid crystal panels.
Unlike voltage-driven liquid crystal elements, organic EL elements are so-called current-driven elements. When driving organic EL elements, it is necessary to generate current, not voltage, in accordance with the gray level of a pixel. FIG. 24 shows a related art current generating circuit that generates current.
In FIG. 24, a voltage generating circuit is a current-adding-type D/A converter, which switches on/off transistors 20a to 20f in accordance with 6-bit digital data (D0 to D5) indicating the gray level of a pixel to select elemental currents i1 to i6 and which combines the selected elemental currents to obtain a current lout in accordance with the gray level.